In a known diagnosing method using an endoscope, fluorescent markers having an affinity to lesions, such as those resulting from cancer(s), are introduced into a subject's living tissue, the fluorescent markers are irradiated with excitation light, and fluorescence from the fluorescent markers that accumulate at lesions is detected. In order to improve the accuracy of diagnosis using such a diagnosing method, fluorescent markers are being used that emit specific spectral profiles and that have an affinity to multiple, different substances that appear while a cancer develops in a living tissue. The appearance of the multiple, different spectral profiles in the emission spectra of light emitted by the fluorescent markers is then detected.
For example, a prior art endoscope realizing such an observation technique as described above is disclosed in Japanese Laid Open Patent Application S63-271308, wherein an element having a variable light transmittance is provided in an image pickup unit. One element that is known to have a variable light transmittance is an etalon in which surfaces that have been made highly reflective are separated by a small gap, and the size of the gap is changed in order to select a wavelength that is transmitted by the etalon.
FIG. 17 is a schematic illustration of a prior art etalon. The etalon shown in FIG. 17 consists of two substrates 35X-1 and 35X-2 having reflective coatings 35Y-1 and 35Y-2, respectively, on their facing surfaces with a gap d between the reflective coatings 35Y-1 and 35Y-2. Light incident onto the substrate 35X-1 is subject to multiple beam interference due to the coatings 35Y-1 and 35Y-2. By changing the optical path length within the gap, such as by changing the width of the gap d, the light that emerges from the substrate 35X-2 changes in wavelength. For example, when the width of the gap d in FIG. 17 is changed from a value “a” to a value “b”, the wavelength of maximum transmittance will change from Ta to Tb. The width of the gap can be changed, for example, by moving the substrates, such as by using a piezoelectric element.
FIG. 18 illustrates, for example, the spectral transmittance profile of the prior art etalon shown in FIG. 17, for the situations of the spacing d having a value “a” versus a value “b”.
It is known that a small etalon may be produced by surface micro-machining, and in which the width of a gap may be changed using electrostatic forces.
FIG. 19 schematically illustrates the structure of such a small etalon. The small etalon in FIG. 19 is provided with facing mirrors 35′ X-1 and 35′ X-2 having a high reflectance. Metal coatings or dielectric multilayer coatings are laminated on the facing surfaces of the mirrors 35′ X-1 and 35′ X-2. The mirror 35′ X-1 is coupled at its outer periphery to an elastically deformable hinge 35′ Z-1. The mirror 35′ X-2 is fixed at its outer periphery to a substrate 35′ Y-2. The hinge 35′ Z-1 and the substrate 35′ Y-2 are spaced from each other and coupled at their outer periphery to spacers 35′a, 35′a. In the small etalon in FIG. 19, a microactuator (not illustrated) is used to create an electrostatic force between the facing mirrors 35′ X-1 and 35′ X-2 in order to elastically deform the hinge 35′ Z-1, thereby changing the width of a gap d between the mirrors 35′ X-1 and 35′ X-2. With the width of the gap d being changed, light of a different wavelength is transmitted by the small etalon.
The etalon has a peak transmittance at a wavelength λ when the optical path length of the gap d satisfies the following Condition (1):d=(λ/2)·n (n is an integer)  Condition (1).
It is understood from Condition (1) above that the peak transmittance wavelength λ shifts as the gap d is changed. Since the peak transmittance wavelength shifts as the gap is changed, the peak transmittance wavelength can be readily determined for a given gap d, and vice-versa.
In a fluorescent observation described above using an image pickup unit having an etalon, the gap is controlled based on a reference gap d0, and the peak transmittance wavelength is scanned to detect the fluorescence emitted by multiple fluorescent dyes that emit different spectral profiles within a range of wavelengths scanned by the etalon. Thus, it can be readily determined which fluorescent dye(s) is(are) producing the emitted fluorescence.
In an image pickup apparatus using an image pickup unit having an etalon, multiple images can be captured at a high speed using light having different wavelengths, and the image information may be analyzed, for example, to diagnose a cancer that has developed in living tissue but is difficult to detect by other means in its early stage.
Etalons have different operational accuracies that result from production errors. Therefore, an image pickup unit having an etalon and an image pickup apparatus using such an image pickup unit needs to be individually calibrated, based on a reference gap, for operational accuracy.
Further, in an image pickup unit having an etalon and in an image pickup apparatus using such an image pickup unit, the etalon will be influenced by the work environment. Therefore, the etalon is subject to changes in operational accuracy. For example, when an image pickup unit having an etalon is provided in an endoscope at the insertion end, the etalon exhibits different scanning performance before versus after the insertion end reaches the site to be examined. This presumably occurs because the temperature of the endoscope insertion end changes upon the endoscope end being inserted at the examination site. Therefore, it is desirable to set the etalon for a reference gap (i.e., to calibrate it for operational accuracy), after the endoscope insertion end has been inserted and immediately before the peak transmission wavelength of the etalon is actually scanned.
The above-mentioned Japanese Laid Open Patent Application S63-271308 fails to disclose any means or method for resolving these problems.